1. Field of the Invention
The present invention relates to communication systems, and, in particular, to fault-tolerant switches used to route signals in optical telecommunication systems.
2. Description of the Related Art
Optical cross-connect systems will play a very important role in high-speed communication networks as optical technology is deployed. An optical channel can carry terabits of information per second. On the other hand, as bit rate increases, tolerance for signal interruption significantly decreases. For example, a 50-ms interruption in a DS-1 signal (having a bit rate of 1.544 Mb/s) will cause a loss of 77,200 bits of information. However, when an optical channel is carrying an OC-192 signal (having a bit rate of 9.92 Gb/s), a similar 50-ms signal interruption will cause a loss of 496 million bits! If an optical channel is employing dense wavelength division to multiplex N different wavelengths, each carrying 9.92 Gb/s seconds, then the loss of information bits further increases N-fold. In many applications, it is very important that there are no transmission errors, even in the presence of single-point equipment failures.
FIG. 1 shows a high-level block diagram of an optical cross-connect system 100, comprising an optical cross-connect switch fabric 102 configured to a controller 104, 1024 STS-48 input interfaces 106, and 1024 STS-48 output interfaces 108. Each input interface 106 transmits its input signal to switch fabric 102 via a different input optical fiber 110. Similarly, switch fabric 102 transmits a different output signal to a corresponding output interface 108 via a different output optical fiber 112. Each of the 1024 input signals can be connected to any of the 1024 output signals in a non-blocking manner. Controller 104 provides the configuration information to switch fabric 102 such that each signal is routed to the correct output interface 108.
As evident from FIG. 1, if any fiber (110 or 112) or any switching element (within switch fabric 102) fails, the corresponding signal(s) will be interrupted until the fault is repaired. In order to provide error-less transmission in the event of such a single-point failure, traditional fault-tolerance techniques require a triplicated switch fabric.
FIG. 2 shows a block diagram of an optical cross-connect system 200 configured with a conventional triplicated switch fabric to provide fault tolerance for error-less switching. In particular, optical cross-connect system 200 comprises three optical cross-connect switch fabrics 202, each configured to a controller 204, 1024 STS-48 input interfaces 206, and 1024 STS-48 output interfaces 208. As shown in FIG. 2, the input signal from each input interface 206 is transmitted to each of the three switch fabrics 202 via three different input fibers 210 and each output interface 208 receives an output signal from each of the three switch fabrics 202 via a different output fiber 212. According to this traditional fault-tolerance technique, each output interface 208 implements a fault-detection and error-recovery scheme to produce an appropriate output signal.
FIG. 3 shows a block diagram of the components used to implement the fault-detection and error-recovery scheme within each output interface 208 of FIG. 2. In particular, each output signal is routed via output optical fiber 212 through a buffer 301 to a selector 303. In addition, there are three independent monitors 305, each of which monitors the corresponding output signal for failure (e.g., a signal open or a signal short). A voter 307 monitors the status of the signals and causes selector 303 to select a healthy signal from the three incoming signals as the output signal for the corresponding output interface 208. The health of a signal is determined by majority voting. In the event of failure in one signal, information from two healthy signals will match and voter 307 will cause selector 303 to select one of the healthy signals for transmission.
As indicated by FIGS. 2 and 3, traditional fault-tolerance techniques require three switch fabrics with three different sets of incoming and outgoing fiber cables. In addition, for error-less switching, the traditional techniques require sufficient storage (i.e., buffering) for each of the three signals, to enable a fault to be detected before the selected output signal leaves the system. As such, these traditional fault-tolerance techniques require a relatively large overhead to achieve error-less switching.